Memory devices used for computer data storage can be classified into two categories: volatile memory and non-volatile memory. For volatile memory, such as DRAM, maintaining device states and preserving stored information requires a constant supply of power. Any interruption of power will result in loss of stored information. Preventing such loss requires the use of back up batteries or other energy storage devices, which may be expensive, bulky and difficult to maintain.
Non-volatile memory, by comparison, does not need power to maintain its information content. However, non-volatile memory may not be as economical or efficient or fast as volatile memory, and has accordingly not replaced volatile memory as a viable data storage alternative. Nevertheless, in certain circumstances, it can be useful to back up volatile memory with non-volatile memory, for example to avoid catastrophic data loss in the event of power outage. Data thus backed up in non-volatile memory is preserved despite power disruptions, and can be copied back into the volatile memory when normal operation resumes.
It may be inefficient or impractical to back up all volatile memory data all the time using non-volatile memory. More practical approaches rely on selective storage, for example of critical data only; or on detecting a potential power loss in advance and then backing up any critical data, or data that is intermediate to a currently-executing process, while some power still remains, or while a small, economical amount of power can be provided.
There are many types of non-volatile memory. One common type is termed “flash” memory, and relies on charge storage in gates of floating-gate transistors. The charge storage is persistent and interruptions in power have no short term impact on the information content, endowing the memory with its non-volatile character. Individual flash memory cells, comprised of one or more floating-gate transistors, can be configured to store a single binary value (single-level cells, or SLCs), or multiple binary values (multi-level cells, or MLCs). The flash memory chip can comprise millions, or billions, of such cells, and is currently available in numerous formats, such as 2 gigabit (Gb), 4 Gb, 8 Gb, and so on. The chips themselves can be combined in various architectures in a memory module, to be accessed by way of a flash memory controller that selectively issues memory accesses commands using control and address signals to the flash memory chips for retrieval or storage of data based on the needs of the host device.